Process for preparing triuret

ABSTRACT

A process for the preparation of triuret by the pyrolysis of a urea-containing reactant system in the presence of an inert liquid carrier at a temperature of from about 112* to about 140*C. The formation of cyanuric acid is minimized by carefully controlling the urea concentration and temperature of the pyrolysis reaction mixture.

United States Patent 11 1 [111 3,862,223

Beale, Jr. Jan. 21, 1975 PROCESS FOR PREPARING TRIURET 1,068,693 11/1959 Germany [75] Inventor: Alvin F. Beale, Jr., Lake Jackson,

T Primary Examiner-Bernard Helfin Assistant Examiner-Michael W. Glynn [73] Asslgneez The Dow Chemical Company,

Midland, Mich- Attorney, Agent, or Fzrm Gary D. Street [22] Filed: Apr. 2, 1973 ABSTRACT [21] Appl. No.: 347,237

A process for the preparation of tnuret by the pyrolysis of a urea-containing reactant system'in the pres- [52] US. Cl 260/553 B ence f an inert liquid carrier at a temperature f f [51] Int. Cl. C07c 127/24 about 1 12 to about 140 The formation of cyanuric Fleld 0f Seal'CIl R, B acid is minimized arefully ontrolling the urea concentration and temperature of the pyrolysis reac- [56] References Cited tion mixture FOREIGN PATENTS OR APPLICATIONS 10/1970 Belgium 11 Claims, 1 Drawing Figure Reacf/on i em oer'o/urq C PATENTEUJANZIVIQIB /26 /34 Reac 7 /0 I? fem oero/ure C PROCESS FOR PREPARING TRIURET BACKGROUND OF THE INVENTION The present invention relates to the preparation of triuret by the pyrolysis of a urea-containing reactant system.

Various methods for the preparation of triuret are known in the art and are summarized at pages l7()l 74 of an article Biuret and Related Compounds" pub lished in Chemical Reviews, 56, p. 95-l97 (I956). With respect to the various methods for preparing triuret set forth in this review article, it was indicated that the deamination of urea by thionyl chloride affords up to 30 percent yield triuret and thus appears to be the most convenient method of preparing triuret. There are, however, many disadvantages associated with the known methods, the principal disadvantage being the difficulty in converting the starting material to the desired triuret product.

The art also teaches that in the preparation of products such as biuret and melamine by the direct pyrolysis of urea at temperatures ranging from 130 to 215C. and higher, small amounts of triuret are formed as a byproduct. See, for example, U.S. Patent Nos. 2,145,392; 2,370,065, and 2,918,467. However, there has been no mention of the specific preparation of triuret by pyrolysis of a urea-containing reactant system. In such prior art urea pyrolysis reactions, it is known that higher temperatures in the abovestated range favor the formation of cyanuric acid and higher total cyanuric acid plus triuret content in the pyrolysis product while the lower temperatures favor the formation of a biuret rich pyrolysis product having a low cyanuric acid and triuret content.

Although the higher pyrolysis temperatures favor the predominate formation of cyanuric acid and the coproduction of small amounts of triuret, attempts to increase the triuret content of the pyrolysis product by continued pyrolysis attack on the urea have been unsuccessful due to the dominate formation of cyanuric acid. Another major difficulty is that as the pyrolysis attack on urea continues and the urea content of the reaction mass is decreased, usually to a range of between about 20 to about 40 percent, the eutectic point of the reaction mass is approached and a plastic sticky mass begins to form. This sequence of physical change soon causes gumming and sticking of the reaction mass to the reaction equipment in sufficient quantities which effectively block further attempts to agitate the reaction mass and continue the pyrolysis reaction. Although heating of the thickened reaction mass at still higher temperatures serves to increase the fluidity of the reaction mass, no increase in the triuret content of the product is achieved due to the dominate formation of cyanuric acid.

Consequently, it would be desirable to provide for the preparation of triuret in higher yields than heretofore possible. In view of the ready availability of urea and urea pyrolyzate products, it would be especially desirable to provide a process for the preparation of triuret from a urea-containing reactant system while, of course, avoiding the heretofore mentioned difficulties. It is therefore a principal object of the present invention to provide a process for the preparation of triuret from a urea-containing reactant system, in which the proportion of the pyrolysis of urea to biuret is diminished and the decomposition of area or other urea au- BRIEF SUMMARY OF THE INVENTION By the present invention, a product mass having a high triuret content is produced by controllably adding urea to a biuret feedstock in the presence of an inert carrier liquid and maintaining, with agitation, the resulting reaction mixture at a temperature of from about l l2 to about l4()C. for a period of time sufficient to allow for the substantial conversion of the reactants to triuret. Following the reaction period, the solid reaction mass is separated from the inert carrier liquid by conventional liquid-solid separatory procedures and subsequently mixed with hot water, e.g., about 65C. Filtration of the resulting aqueous mixture affords recovery of the insoluble triuret product.

In carrying out the process, the urea addition is controlled so as to maintain the urea concentration of the reaction mixture between about 1 and about 20 weight percent and in accordance with the reaction temperature employed. The reaction temperature is generally employed in an inverse, although not proportional, manner to the urea concentration of the reaction mixture. An inert gas sparge is usually employed to remove evolved ammonia by-product from the mixture; however, the use of certain inert carrier liquids having a normal boiling point at about the predetermined reaction temperature employed eliminates the need for a separate gas sparge since the vapors from such carriers serve to carry off the ammonia.

BRIEF DESCRIPTION OF THE DRAWING The attached drawing is a graphical presentation defining the relationship of the urea concentration and reaction temperature parameters of the claimed process.

DETAILED DESCRIPTION OF THE INVENTION In the actual practice of one preferred embodiment of the present invention, increments of particulate urea are controllably added to a mixture of a biuret feedstock suspended in an inert liquid carrier and the resulting reaction mixture, i.e., the urea-containing reactant system, heated at a temperature of from about 1 12 to about C. The reaction mixture is heated for a period of time sufficient to effect substantial reaction between the urea and biuret components of the reaction mixture and optimum conversion of the components to triuret.

The biuret feedstock employed in the present invention can be biuret or a urea pyrolyzate containing substantial amounts of biuret and lesser amounts of urea, cyanuric acid, triuret and trace amounts of other urea autocondensation products such as, for example, ammelide. In a preferred embodiment of the present invention, the biuret feedstock employed contains at least about 40 percent biuret. In a further preferred embodiment, the biuret feedstock employed contains major amounts of biuret, i.e., above about 50 percent by weight. In an additional preferred embodiment, biuret itself is employed.

In carrying out the process of the present invention, it is essential that the reaction temperature and the urea concentration of the reaction mixture being pyrolyzed be carefully controlled so as to avoid not only the formation of major amounts of biuret or cyanuric acid but also agglomeration of the reaction mixture. In this respect, when particulate urea is added to a slurry of a solid biuret feedstock in an inert liquid carrier (wherein the urea concentration of the resulting slurry is about 20 weight percent or more, based on solids present), and the resulting slurry is heated at a temperature of about 1 12C., the mixture of suspended urea and biuret melts. Once the mixture has melted. it will supercool upon cessation of heating and will resolidify at about 90-95C. Continued heating of the melted mixture at about 112C. will cause the melted mixture to thicken and soon agglomerate and adhere to the reactor surfaces. However, it has now unexpectedly been discovered that by maintaining the urea concentration of a urea and biuret feedstock slurry reaction mixture below 20 weight percent, preferably at about 18 weight percent or less, agglomeration is avoided at about 112C. By thus decreasing the urea concentration of the slurry reaction mixture as the reaction temperature is increased, the problem of agglomeration is effectively avoided and the reaction between the suspended urea and biuret components to form triuret is favored.

With reference to the drawing of the present invention, the area ABC of the graphical presentation defines the boundaries for the amount of urea which can be present in various urea-containing reactant systems employed in the present invention in accordance with the various reaction temperatures and still avoid forming the undesired agglomerated reaction mass hereinbefore referred to. Line AC of the drawing represents the approximate maximum urea concentration of the reaction mixture and corresponding reaction temperatures which can be employed and still avoid forming the undesired agglomerated reaction mass. The maximum amount of urea which can thus be tolerated in a slurry reaction mixture at a maximum temperature of about 140C. is about 1 weight percent. At temperatures above about 140C., the condensation of triuret to cyanuric acid and other autocondensation products is favored. in a preferred embodiment of the present invention, the urea concentration and reaction temperature parameters of the reaction mixture are employed in a relationship as defined by Area DBC of the drawing. The employment of operating parameters as defined by line DC of the drawing represents an additional preferred embodiment since agglomeration problems are minimized while good conversion of the reaction mass to triuret is attained.

The pyrolysis rate of urea, and thus the rate of reaction between urea and biuret to form triuret, increases as the temperature increases. The pyrolysis of urea to biuret is, however, a faster reaction than the reaction of urea with biuret to form triuret. High concentrations of biuret in the biuret feedstock, and thus in the reaction mixture, and low concentrations of urea in the reaction mixture are therefore preferred. 1n a preferred embodiment, the urea concentration of the reaction mass is maintained at a further preferred embodiment, the urea concentration of the reaction mass is maintained at a concentration of from about 1 to about 8 percent.

The use of an inert liquid carrier has been found to provide at a given temperature a good rate of conversion of the urea and biuret feedstock reactant to triuret while minimizing the formation of other urea autocondensation byproducts. Another advantage of the carrier is that it acts as a heat transfer medium, thereby providing for close control of the reaction temperature; this enables minimization of agglomeration problems as well as by-product formation. A further advantage is the fact that the carrier liquid serves to protect the surfaces of reactors and material handling equipment from direct contact with the reactants or reaction products which could have a corrosive affect thereon.

Representative examples of carrier liquids suitable for use in the practice ofthe present invention includes those materials which have a density less than that of the reactants employed, i.e., the urea and biuret feedstocks and resulting reaction mixtures thereof, and preferably have a freezing or gelling point below about 20C. Such carriers should be substantially a nonsolvent for urea, the biuret feedstocks and the triuret product and be inert to those substances as well as inert to the ammonia evolved during the process.

Operable materials preferably employed as carrier liquids include mineral oils, i.e., petroleum derived oils, and saturated branched-chain and straight-chain hydrocarbons of the alkane series having from 8 to 12 carbon atoms. Alkanes of lower or higher carbon contents can be employed through use of suitable system pressure control or in blends of high and low boiling components. Representative examples of alkane liquid carriers suitable for use in the present invention illustratively include, for example, n-octane, 3- methylheptane, 3-ethylhexane, 2,5-dimethylhexane, 3-ethyl-3-methylpentane, 2,2,4-trimethylpentane, nnonane, 2-methyloctane, 3-ethylheptane, 2,4- dimethylheptane, 3,3,4-trimethylhexane, 4- propylheptane, 2,2,4,5-tetramethylhexane, n-

undecane, n-dodecane, or the like.

Ordinarily carriers having a normal boiling point at about the predetermined reaction temperature are employed. The use of such a carrier liquid at its boiling point virtually eliminates the need for a separate inert gas sparge to remove evolved ammonia by-product gas from the reaction mixture since the vapors from this liquid serve to carry off this byproduct. Once having been removed from the reaction zone this carrier liquid can be condensed, the ammonia separated therefrom and both be recovered. Additionally, a carrier liquid can be selected which has a normal boiling point higher than the reaction temperatures disclosed. The pressure of the system can be controllably reduced below atmospheric pressure until the observed boiling point of the liquid is that of the predetermined reaction temperature. By use of the partial vacuum, the gaseous vapors are actually pulled from the reaction zone thereby further facilitating by-product ammonia removal from the sytem.

Conversely, the boiling point of hydrocarbon carrier liquid can be raised a predetermined amount by con- I trollably applying a superatmospheric pressure to the system.

Boiling point control of the carrier liquid also can be achieved by use of mixtures of lower and higher boiling components.

The relative quantities of the total urea and biuret feedstock mass and carrier liquid to be employed can be varied. For optimum efficiency in handling, heat consumption, reactant contact, product recovery, etc. the relative proportions of the urea-biuret feedstock carrier liquid range on a weight basis from about :10 to about 10:90. Usually, the mixture contains from about 20 to about 50 weight percent of the urea-biuret feedstock mass. At liquid carrier contents of less than about 50 weight percent stirring difficulties sometimes may be encountered. With reaction mixtures containing excessively large amounts of the carrier, i.e., much greater than above 90 percent, increased expense, e.g. heating and handling costs, can be encountered without providing any marked increase in product yield or process efficiency.

The process of the present invention is adapted to be carried out in batch or cyclic-batch operations. In batch operations, urea is added under controlled conditions to a predetermined amount of a biuret feedstock in an inert carrier liquid with agitation. The urea is added at rates, usually incrementally or portionwise, calculated to maintain the urea content of the resulting reaction mixture (based on solids present) at concentrations corresponding to a particular reaction temperature within the boundaries of area ABC, preferably with the boundaries of area DBC, of the drawing. The reaction mixture is maintained at the reaction temperature for a period of time sufficient to effect substantial conversion of the urea and biuret components of the reaction mixture to triuret. The rate and degree of conversion can be determined by intermittent product sampling of the reaction mass and analysis of the same by infrared spectroscopy and by monitoring the evolution rate of by-product ammonia.

The actual reaction period varies with the reaction temperature and employed and the concentration of urea in the reaction mixture. At higher temperatures and lower urea concentrations within the ranges hereinbefore indicated, the conversion to triuret is favored and agglomeration ofthe reaction mass is avoided. The employment of low urea concentrations also favors the reaction of urea with biuret to form triuret and diminishes the rapid condensation. of urea to biuret. In this respect, when urea is added to the biuret feedstock slurry mixture, it quickly melts and adheres to the biuret feedstock particle surface. By maintaining a low level of urea in the reaction mass, the rigidity of the biuret feedstock particles in the reaction mixture is maintained, thus impeding urea migration and condensation to form biuret. Such rigid particles also impede the tendency of the formed triuret to lose ammonia and form cyanuric acid which, upon increase, necessitates a lower reaction temperature and a lower urea content to prevent agglomeration, thereby diminishing the reaction rate to form triuret.

Generally, depending upon the scale of operation and biuret feedstock employed, good conversion of urea and biuret to triuret are obtained in periods of from about 20 hours to about 300 or more hours, usually from about 20 to about 200 hours. When biuret is employed as the biuret feedstock, longer reaction periods of from about to about 50 or more days are required. Following the reaction period, a slurry comprising the liquid carrier and solid reaction mass containing up to about 45 percent by weight triuret, the remainder comprising mainly biuret and small amounts of urea, is recovered. The triuret-containing product mass can be separated from the liquid carrier by conventional solidliquid separatory procedures. The triuret product itself can be obtained by mixing the product mass with hot water and subsequent filtration of the same while hot to remove the insoluble triuret product. The filtrate can be cooled to recrystallize urea and biuret.

The triuret product prepared according to the process of the present invention is a useful component in fertilizer compositions and is useful as a non-protein nitrogen feed supplement for ruminant animals.

The following example is intended to illustrate certain of the several embodiments of the present invention and as such is not intended to be limitative.

EXAMPLE 1 A glass vessel fitted with a bladed turbine type impeller for a stirrer, nitrogen sparge and exhaust tube was used as a reactor. The reactor was heated with a Thermowatch electronic controller. Effluent gases and vapors were vented through a trap and standard sulfuric acid with methyl red indicator.

About 3,256 grams of a white mineraloil and about 361 grams ofa biuret feedstock (urea 8.0 percent; cyanuric acid 10.0 percent; triuret 34.0 percent; biuret 48.0 percent; and trace amounts of ammelide) was charged into the reactor and heated at a temperature of about C. with agitation and under a nitrogen sparge. Any water present boils off during the initial heating. Increments of urea, initially portions of about 3.2 grams, were added to the mixture over a 6 hour period and at a rate calculated to maintain a urea concentration in the reaction mixture of about 6 weight percent (based on solids present). A total of about 32 grams of urea was added during the 6 hour period. After the completion of the urea addition, the reaction mixture was maintained at about 120C. for an additional 18 hour period. The progress ofthe reaction was monitored and the relative composition of the reaction mixture determined by infrared spectroscopy analysis. Following the reaction period, the reaction mixture was found to contain about 43 percent by weight triuret (based on solids), the remainder comprising predominately biuret and small amounts of urea.

Additional portions of urea (about 5 gram increments) were added to the above reaction mass over a period of about 6 hours until a total of about 38.2 grams of urea had been added. The rate of urea addition was such that the total urea content of the reaction mass exceeded 6 weight percent. Following the completion of the urea addition, the reaction mass was maintained at the reaction temperature for a period of about 18 hours. The reaction mixture was found to contain about 34 percent by weight triuret, thus indicating that the rate of urea addition, i.e., the urea concentration of the reaction mass, directly affects the triuret yield under similar reaction temperatures and periods.

The product thus obtained is purified by mixing the same with hot water at a temperature of about 65C. The resulting mixture is filtered while hot to recover the insoluble triuret product.

The biuret feedstock employed above is obtained by adding urea to a biuret pyrolyzate product of the prior art (usually containing about 12% urea, 60% biuret, 14% triuret and 14% cyanuric acid) in an inert liquid carrier over a period of one week. During such addition, the urea content of the pyrolysis mixture is maintained at or below about 18 weight percent (based on solids) and the pyrolysis temperature is maintained between about 112 and C.

EXAMPLE 2 In additional operations employing similar procedures and equipment as in Example 1, about 3374 grams ofa white mineral oil and about 508 grams of biuret are charged into the reactor and heated at a temperature of about 1 l4-l C. with agitation and under a nitrogen sparge. Increments of urea, calculated to maintain the urea concentration of the reaction mixture at about 1-3 percent, are added to the reaction mixture over a period of about 49 days. During the course of the reaction, the pyrolysis temperature is gradually increased until the pyrolysis temperature is about 132C. at the completion of the reaction. Analysis of a pyrolysis mixture sample indicates the triuret concentration to be about 38 percent by weight.

In other operations, a number of runs are made wherein the urea content of a reaction mixture and the reaction temperature thereof are varied in order to determine the urea concentration and reaction temperature combination at which a plastic, sticky reaction mixture begins to form. In such operations, a slurry of a biuret feedstock suspended in an inert liquid carrier is heated and maintained at a predetermined reaction temperature and increments of urea are added thereto until the reaction mixture becomes plastic and sticky. In such operations it is found that at temperatures of about 112, 126, 132 and 140C, respectively, the maximum amount of urea which can be tolerated in the reaction mixture while avoiding the formation of a sticky reaction mixture is about l8, l4, 6 and 1 weight percent or less, respectively.

The invention has been described with respect to certain preferred embodiments and various modifications. Variations thereof will become obvious to persons skilled in the art. lt is therefore to be understood that such modifications and variations are to be included within the spirit and scope of this invention.

1 claim:

1. A process for preparing triuret which comprises providing a reaction mixture of urea, a biuret feedstock and an inert carrier liquid and heating the reaction mixture at a temperature of from about 112 to about 140C. for a period of time sufficient to effect a substantial conversion of the reaction components to triuret. while maintaining the relationship of the urea concentration and the reaction temperature ofthereaction mixture within the boundaries of ABC of the drawing.

2. The process according to claim 1 wherein the reaction temperature and the urea concentration of the reaction mixture are approximately that as defined by the line AC of the drawing.

3. The process as defined in claim I wherein the biuret feedstock has a biuret content of at least about 40 percent. I

4. The process as defined in claim 1 wherein the biuret feedstock is predominately biuret.

5. The process as defined in claim 1 wherein the biuret feedstock is biuret.

6. The process as defined in claim 1 wherein the inert carrier liquid is a mineral oil, or a straightor branchedchain alkane having from 8 to 12 carbon atoms.

7. The process as defined in claim 1 wherein the inert carrier liquid is a mineral oil.

8. The process as defined in claim 1 wherein the urea concentration of the reaction mixture and the reaction temperature are maintained within the boundaries of area DBC of the drawing.

9. The process as defined in claim 1 wherein the urea concentration and reaction temperature ofthe reaction mixture are approximately that as defined by line DC of the drawing.

10. The process as defined in claim 1 wherein the urea concentration of the reaction mixture is maintained at a concentration of from about 1 to about 12 weight percent.

11. The process as defined in claim 1 wherein the urea concentration of the reaction mixture is maintained at a concentration of from about 1 to about 8 weight percent. 

1. A PROCESS FOR PREPARING TRIURET WHICH COMPRISES PROVIDING A REACTION MIXTURE OF UREA, A BIURET FEEDSTOCK AND AN INERT CARRIER LIQUID AND HEATING THE REACTION MIXTURE AT A TEMPERATURE OF FROM ABOUT 112* TO ABOUT 140*C. FOR A PERIOD OF TIME SUFFICIENT TO EFFECT A SUBSTANTIAL CONVERSION OF THE REACTION COMPONENTS TO TRIURET, WHILE MAINTAINING THE RELATIONSHIP OF THE UREA CONCENTRATION AND THE REACTION TEMPERATURE OF THE REACTION MIXTURE WITHIN THE BOUNDARIES OF ABC OF THE DRAWING.
 2. The process according to claim 1 wherein the reaction temperature and the urea concentration of the reaction mixture are approximately that as defined by the line AC of the drawing.
 3. The process as defined in claim 1 wherein the biuret feedstock has a biuret content of at least about 40 percent.
 4. The process as defined in claim 1 wherein the biuret feedstock is predominately biuret.
 5. The process as defined in claim 1 wherein the biuret feedstock is biuret.
 6. The process as defined in claim 1 wherein the inert carrier liquid is a mineral oil, or a straight- or branched- chain alkane having from 8 to 12 carbon atoms.
 7. The process as defined in claim 1 wherein the inert carrier liquid is a mineral oil.
 8. The process as defined in claim 1 wherein the urea concentration of the reaction mixture and the reaction temperature are maintained within the boundaries of area DBC of the drawing.
 9. The process as defined in claim 1 wherein the urea concentration and reaction temperature of the reaction mixture are approximately that as defined by line DC of the drawing.
 10. The process as defined in claim 1 wherein the urea concentration of the reaction mixture is maintained at a concentration of from about 1 to about 12 weight percent.
 11. The process as defined in claim 1 wherein the urea concentration of the reaction mixture is maintained at a concentration of from about 1 to about 8 weight percent. 